The present invention relates to the art of medical diagnostic imaging. The invention finds particular application in conjunction with X-ray imaging apparatus and will be described with particular reference thereto. The invention will also find application in other imaging systems where control of exposure times are important, such as, for example, nuclear or gamma camera type systems, or the like.
The typical X-ray imaging apparatus includes an X-ray generator that radiates an X-ray beam in a direction towards a patient disposed between the X-ray generator and an X-ray film screen. The film is usually contained in cassette that is disposed adjacent an ion chamber. The X-ray beam is developed at the X-ray generator by applying a high voltage between an X-ray tube anode and an X-ray tube cathode, sometimes referred to as an electron emissive filament. When a positive large voltage is applied to the X-ray tube anode, the cathode filament is heated causing electrons to be scattered randomly therefrom. An electron beam focusing cup associated with the cathode concentrates the electrons from the cathode to impinge at a focal spot on the anode to, in turn, produce an X-ray beam emitting from the focal spot.
It is known that the energy or penetrating power of the X-ray beam generated by the X-ray tube is proportional to the kilovoltage kV that is applied between the anode and cathode of the X-ray tube. Also, the quantity or intensity of the X-ray photons is proportional to the electron beam current mA that flows between the anode and the cathode of the X-ray tube. Both the X-ray tube kV and mA are exposure control factors that are selected by an imaging technician before commencing an exposure.
One other parameter that is selectable by the imaging technician is the exposure time of the X-ray beam on the patient. Precise exposure control is critical to produce good, clear X-ray images. In addition, since over-exposure of patients to X-ray beams could be harmful to the patient, precise exposure control is critical.
In the past, analog automatic exposure control systems have been used in X-ray imaging apparatus to extinguish the X-ray beam based on a comparison between an analog feedback signal and various control and other parameters selected by an imaging technician. Analog automatic X-ray exposure control systems, however, have met with limited success.
One problem with conventional analog automatic exposure control systems has been their limited dynamic range, especially when interfaced with standard type ion chambers typically found in most X-ray imaging devices. The typical analog automatic exposure control system includes an integrator circuit disposed at the ion chamber for developing an X-ray power integration signal. The signal dynamic range, however, is limited by the power supply of the integrator, typically plus/minus 15 volts. Accordingly, it becomes very difficult to accommodate a wide range of X-ray film/screen speed combinations due mainly to signal saturation in the integrator.
Another problem with conventional analog automatic exposure control systems is their poor signal-to-noise ratio at low signal levels. This, in turn, causes a significant film density variation for high kV imaging procedures in normal use. The poor signal to noise ratio of the conventional analog systems is due mainly to comparator noise at the X-ray generator and, in addition, to noise caused by analog transmission of the integrator signal typically long signal cables extending between the ion chamber and the X-ray generator.
Lastly, in connection with the shortcomings of the conventional analog automatic exposure control systems, another problem is the difficulty in adjusting those systems to provide for a wide range of short exposure time compensation. In that regard, precise pre-termination techniques require an enhanced level of adjustability to accommodate the anticipated range of ion chamber response time delays and generator exposure termination delays that one would expect to face when using an X-ray imaging apparatus on a wide range of body parts with multiple patients. Conventional analog short exposure time compensation circuits include a differentiator with a potentiometer and a summing amplifier to compensate the X-ray imaging apparatus for short exposure times. These circuit typically provided only a modest level of adjustability. Also, access to the potentiometer and manual manipulation thereof to adjust the X-ray pre-termination trip point was time consuming and inconvenient.
It would, therefore, be desirable to provide a digital automatic X-ray exposure control circuit that is relatively immune to signal noise and is operable over a wide dynamic range to accommodate many X-ray film and film speed combinations.
It would further be desirable to provide such a digital exposure control system in order to improve the signal-to-noise ratio of the imaging apparatus at low signal levels. This would allow for longer signal cable lengths between the X-ray generator and the ion chamber.
Still further, it would be desirable to provide a digital exposure control system that can accommodate a wide range of ion chamber response time delays and X-ray generator exposure termination delays. It would be desirable to provide for digital pre-termination trip points to effect short time compensation.